Hi All,
I'm new to this site and wondering if anyone has ideas on this,
My 94 k3500 7.4l has no power to the fuel pump fuse. The fuel pump relay clicks when ignition is turned on but no power to the fuse. If i spray fuel in the TBI's it runs good then obviously quits. I'm new to this engine so any help would be greatly appreciated! Was thinking of trying to put power to one side or the other of the fused wire to see if the pump would fire up...not sure if this could cause issues with whatever should be sending power to the fuse??

Thank you

 

At the fuel pump relay connector -

1) Check for battery voltage on the Orange wire at the relay. This is a wire that’s hot at all times and provides power to the fuse once the relay is activated. It should be within 0.2 volts of battery voltage.

If battery voltage is present at the Orange wire, use a jumper wire to bypass the relay and jump the Orange and gray wires together - if there’s now power to the fuse/fuel pump, most likely the relay needs replacement due to worn contacts.

If battery voltage is NOT present at the Orange wire, most likely the fusible link that provides power to it needs replacement. Cut the Orange wire on the engine side of the fusible link and jump it to the positive battery terminal - voltage present at the relay means the fusible link needs replacement. You can replace it with fusible link wire or with a standard inline fuse.

 

Hi,
thank you. I do have power to the orange wire on the relay. Voltage to the orange wire is exact with the battery voltage. I picked up a new relay today and still no power to the fuse.

I have heard the fuel pump fuse gets power from the relay during the crank cycle. the relay energizes during crank from the CRKfuse (5A), not sure where that would be.
after start up power for the fuel pump fuse comes from the fuel pump oil pressure switch. Has anyone heard of this before?

Thank you

 

I apologize for failing to mention this in my previous post -

At key on, the ECM will activate the FP relay for 2 seconds to prime the fuel line, then it will deactivate the relay until it detects a cranking signal, at which point it activates the relay to continue supplying power to the FP.

If you checked for power @ the FP fuse more than 2 seconds after turning on the ignition, there’s not supposed to be power there.

There is no direct fused circuit for the coil side of the FP relay. It is powered internally by the ECM with a direct ground to the engine block.

Feeling the relay click indicates the ECM is capable of supplying adequate power to activate the relay and that the ground is also in tact. Even if the voltage is weak the relay is still being activated, so further pursuit of the control side of the relay is not necessary at this point in time.

The oil pressure switch provides back up power to the FP in case the primary circuit fuse blows. The oil pressure switch closes when oil pressure is present (engine already running), however it will not provide power to the fuel pump during cranking because the cranking oil pressure isn’t adequate to close the switch.

Both the FP relay (switch side) and the oil PSI switch are powered from the same fusible link wire on the passenger side firewall. Power at the orange wire of the FP relay is a good indicator the power supply for the oil PSI switch is also intact.

If you’re questioning the relay at all (switch or coil), unplug it and jumper the orange and gray wires - power should now be present at the fuse.

Here’s the diagram.



FP Relay marked by white arrow
Oil PSI switch marked by black arrow.

 

Good morning,

Thank you for this, im starting to understand it a bit better. I did replace the relay last night but did not check the FP fuse within a few seconds. I will try these things you have suggested tonight and tomorrow. So if i jumper the relay i should have constant power to the FP fuse and hear it run as long as the ignition is on? If there is power and cant hear the pump then likely a bad FP ground or the pump itself? Thank you again for all you time and knowledge!

 

The orange wire is a direct battery feed - the ignition doesn’t need to be on.

Jumping the FP relay will allow the pump to run with the key off, assuming the pump and related circuits are functional.

 

Good morning Gumby22!

This morning i jumpered the relay as you suggested and i do have power to the FP fuse! Still nothing from the pump however. I am taking the box off and will check the fuel pump and wiring, regardless at this stage i will replace the pump. I will let you know how it goes. Thank you again for sharing your knowledge!

 

If you really want to test the integrity of the FP circuits, then once you have access to the FP connector, use jumper wires to install a halogen headlight bulb in place of the FP and see how brightly it glows (test it at the battery first for reference).

If it glows at full brightness, the circuits are good. If it’s glowing dimly (or not at all), there’s a circuit issue (and from experience I can tell you it’s 50-50 whether it’s the power or ground circuit).

 

Shes running!!! I bought the truck from a neighbor and the truck has sat since 2007. Drained all the old fuel, put in the new pump and filter and she fired right up and runs great!! I cannot thank you enough for all your help!! Thank you sooooo much! everyone I talked to around here had no idea how the system actually worked. Hopefully if I need some expertise in the future you'll find my post!! thank you again!!